Sculpting Shape Keys with Multires Modifier?
I believe shapekeys can only modify the base mesh. Changes made to higher multires levels are not affected by shape keys. If in the multires settings Sculpt level is 0, you are sculpting the base mesh. When in the video he says &quot;apply sculpt to base mesh instead of multires layers&quot;, he means you should sculpt the base mesh only, not higher multires levels.
Friend3D, by saying multires with shapekeys worked for him, perhaps means that modifying base mesh with shape keys didn't break higher multires levels in his test. It always used to break when that was done before Blender 2.9, though I'm not certain it doesn't sometimes still do that.